Such a unit generally comprises an electric drive motor together with a speed reducing mechanism, in particular a mechanism of the worm type enclosed within the hollow casing by the cover plate.
The casing includes within its hollow interior, as is well known, a toothed wheel which is driven in rotation by the worm, and which actuates, via an eccentric gudgeon pin carried by this toothed wheel, a crank and connecting rod system which operates an output shaft, in such a way that the latter moves in alternating rotary motion. The output shaft extends through the base of the casing for connection with the screen wiper to be driven by it.
The crank and connecting rod system consists of an arm which is connected at one of its ends to the eccentric gudgeon pin carried by the toothed wheel. At its other end, this arm has a toothed sector which meshes with a second toothed sector. This second toothed sector is in turn fixed to the output shaft. In order that the two toothed sectors will remain in continuous engagement, they are linked together on either side by means of balance members, which also constitute the crank of the crank and connecting rod system.
However, such a motorized reduction gear unit has to be able to transmit very large forces. The crank and connecting rod system is subjected not only to forces transmitted by the toothed wheel in the axis of rotation of the connecting rod, which can cause the crank and connecting rod system stall, so as to give rise to axial displacement of the pivot of that system, but also to axial forces on the axis of the gudgeon pin, or on the axis of the output shaft itself.
It has previously been proposed, in a previous patent application of the present Applicants, namely French published patent application FR 2 667 670A, to immobilize against axial movement the eccentric gudgeon pin which is carried by the toothed wheel, by locating it against the cover plate by means of a friction plate which is fixed against the inner surface of the cover plate, and on which the end of the gudgeon pin engages. The same document also proposes the provision of a friction ring surrounding the end of the output shaft and disposed against the inner surface of the cover plate, being in engagement on one of the balance members that surround the output shaft.
These expedients avoid the situation in which the eccentric pin or the output shaft impact on the inner surface of the cover plate accidentally due to axial forces, so giving rise to noise. However, this problem still remains in connection with the crank and connecting rod system.